Various methods have been proposed for producing cyclohexenes using as starting materials monocyclic aromatic hydrocarbons. For example, it has been proposed to use (1) a method using water, an alkali agent and a catalyst composition containing a member selected from the Group VIII elements of the Periodic Table [Japanese Patent Publication (Kokoku) No. 56-22850/1981]. Further, it has been proposed to use (2) a method in which a reaction is carried out in the presence of a ruthenium catalyst and a neutral or an acidic aqueous solution containing a salt of a cation of at least one member selected from the group consisting of Group IA and Group IIA metals of the Periodic Table and manganese [Japanese Patent Publication (Kokoku) No. 57-7607/1982]. Still further, it has been proposed to use (3) a method in which a reaction is carried out in the presence of a ruthenium catalyst dispersed in silica gel derived from a hydrolysis product of a silicon alkoxide, and water [Japanese Patent Publication (Kokoku) No. 60-59215/1985]. Still further, it has been proposed to use (4) a method in which a reaction is carried out in the presence of a catalyst comprising ruthenium supported on barium sulfate, water and an additive [Japanese Laid-Open Patent Application (Kokai) No. 61-40226/1986]. Still further, it has been proposed to use (5) a method in which a reaction is carried out in the presence of a catalyst comprising ruthenium supported on a compound containing a rare earth element, water and an alkali agent [Japanese Patent Publication (Kokoku) No. 1-29174/1989]. Still further, it has been proposed to use (6) a method in which a reaction is carried out in the presence of metallic ruthenium particulates, zirconium oxide or hafnium oxide, and water [Japanese Laid-Open Patent Application (Kokai) No. 62-81332/1987]. Still further, it has been proposed to use (7) a method in which a reaction is carried out in the presence of a ruthenium catalyst using as a starting material a monocyclic aromatic hydrocarbon which substantially does not contain a sulfur compound [Japanese Laid-Open Patent Application (Kokai) No. 60-255738/1985]. Still further, it has been proposed to use (8) a method in which a reaction is carried out in the presence of a ruthenium catalyst and water in an atmosphere which does not cause iron to be deposited on the catalyst [Japanese Laid-Open Patent Application (Kokai) No. 62-67033/1987]. In all of these methods, catalyst slurries prepared by dispersing or dissolving a ruthenium catalyst and various types of additives in water are brought into contact with monocyclic aromatic hydrocarbons and hydrogen by mixing in a liquid phase, thereby obtaining cycloolefins.
When a practical process for continuously producing cycloolefins is designed according to these conventional methods, it is requisite to effect complete separation between a catalyst slurry comprised of a ruthenium catalyst and water (hereinbelow frequently referred to simply as "aqueous phase") and an oil phase containing a partial hydrogenation reaction product and an unreacted monocyclic aromatic hydrocarbon (hereinbelow frequently referred to simply as "oil phase"). With such a process, if components of the aqueous phase, for example, excess amounts of a catalyst and/or an additive (such as a solid material, an alkaline material, or an acidic material which is added for improvement and stabilization of reaction performance.) get mixed into the oil phase, problems, such as clogging of process pipes or corrosion of conventionally used apparatus materials, would occur due to the mixed components. Such problems can be solved to a certain extent by providing, for example, a filtering device or a washing device for removing the mixed components. However, in the commercial practice, facilities and operations therefor are inevitably accompanied with difficulties. Further, in a continuous partial hydrogenation reaction as well, it is apparent that when excess amounts of a catalyst and/or an additive get mixed, and even when gradually get mixed into the oil phase, and flow away, some measures and facilities must be provided for keeping the reaction system stable for a prolonged period of time.
Accordingly, from a commercial viewpoint, means for preventing components of the aqueous phase from excessively getting mixed into the oil phase is strongly desired.
The term, "excess" used herein means an amount in excess of the solubility of components of the aqueous phase in an oil phase under partial hydrogenation reaction conditions or phase separation conditions (for example, the temperatures employed, and the composition of the oil phase generally comprising a reaction product and an unreacted starting material). As a practical matter, however, problems arise when excess amounts of several times the solubility get mixed. Therefore, more specifically, the term "excess" used herein means several or more times the solubility. For example, according to the study of the present inventors, when a partial hydrogenation reaction of benzene is conducted at a hydrogen pressure of 50 kg/cm.sup.2 G and at 150.degree. C. using as an aqueous phase a catalyst and an 18 % by weight aqueous ZnSO.sub.4.7H.sub.2 O solution (which is an additive to be added for improvement and stabilization of a reaction yield) to thereby obtain a reaction mixture comprised of 50 mole % of benzene and 50 mole % of a mixture of cyclohexene and cyclohexane, the solubility of water in the oil phase is about 1% by weight, and the solubility of ZnSO.sub.4 in the oil phase is 1 ppm or less (the solubility thereof in water dissolved in the oil phase is 100 ppm or less). In this instance, the meaning of "excess" is several times, for example, two or three times the above-mentioned solubility of about 1% by weight with respect to water and 1 ppm with respect to ZnSO.sub.4. In addition, it is noted when the above-mentioned excess mixing of water and ZnSO.sub.4 occurs, the mixing of solid, components of the aqueous phase into the oil phase is also observed in most cases.
From this viewpoint, a review is made of the conventional technologies. For example, with respect to the above-described prior art methods (1) to (6), although there is a description regarding a continuous reaction according to a liquid phase suspension method, only a batch reaction is conducted in the Examples thereof and there is no description regarding an attempt to almost completely separate a catalyst slurry from an oil phase, which separation is aimed at by the present invention.
Further, the above-described prior art methods (5) to (8) are the methods developed by the present inventors themselves. Especially in the methods (7) and (8), the partial hydrogenation reaction of monocyclic aromatic hydrocarbons is continuously carried out using a catalyst slurry comprised of a ruthenium catalyst and water. In the prior art methods (7) and (8), continuous reaction was actually performed, and partial hydrogenation reaction was successfully performed over a period of 100 to 500 hours at relatively stable reaction performance. However, there is no description regarding materials having gotten mixed into the collected oil phase and the amounts thereof, and no study has been conducted with respect to almost complete separation of the catalyst slurry from the oil phase, which separation is aimed at by the present invention.